Which of the following compounds has the highest octane number: A) 2,4-dimethylpentane B) 2,2,4-trimethylpentane C) Octane D) 2-methylpentane?

The octane number of petrol is a measure of its resistance to knocking or pre-ignition in an internal combustion engine. A higher octane number indicates better fuel quality and less knocking.

The octane number is influenced by the structure of the hydrocarbon molecules:

• Straight-chain alkanes have low octane numbers and are prone to knocking.
• Branched-chain alkanes have higher octane numbers and are more resistant to knocking.
• The more highly branched an alkane is, the higher its octane number.

Let's analyze the given options:

• A) 2,4-dimethylpentane: This is a branched alkane with a 5-carbon main chain and two methyl branches.
• B) 2,2,4-trimethylpentane: This is a highly branched alkane with a 5-carbon main chain and three methyl branches. This compound is also known as isooctane and is assigned an octane rating of 100, serving as the reference standard for octane numbers.
• C) Octane (n-octane): This is a straight-chain alkane with 8 carbons. Straight-chain alkanes have very low octane numbers.
• D) 2-methylpentane: This is a branched alkane with a 5-carbon main chain and one methyl branch.

Comparing the degree of branching:

• n-octane (C) has no branches.
• 2-methylpentane (D) has one branch.
• 2,4-dimethylpentane (A) has two branches.
• 2,2,4-trimethylpentane (B) has three branches.

Since 2,2,4-trimethylpentane is the most highly branched alkane among the options, it will have the highest octane number.

The final answer is $\boxed{\text{B}}$.